If air is modeled as an ideal gas the only internal energy is kinetic and we can use:

[itex]K = \frac{3}{2} n R T[/itex]

(where R is the universal gas constant = 8.314)

But how do we find the number of moles for dry air? We must use the equation n=m/M. Here M is the molar mass (atomic number). But air is not a single gas, it is composed of several different gases. So how do we know its molar mass?? Do we have to use the atomic number of oxygen or nitrogen or carbon?

I appreciate any advice. If I've gone wrong anywhere else I would appreciate if somebody lets me know.

And why can't we just use Kint=3/2nR*T? (in an ideal gas we only consider the kinetic since molecules interact only by collisions)

Cv is heat capacity at constant volume. You cannot use Cv = 3R/2 because the diatomic molecules have rotational kinetic energy as well as translational kinetic energy resulting in 5 degrees of freedom. (It does not have vibrational kinetic energy at this temperature for reasons having to do with quantum mechanics). So U = n(5/2)RT

Cv is heat capacity at constant volume. You cannot use Cv = 3R/2 because the diatomic molecules have rotational kinetic energy as well as translational kinetic energy resulting in 5 degrees of freedom. (It does not have vibrational kinetic energy at this temperature for reasons having to do with quantum mechanics). So U = n(5/2)RT

I'm sure you are right but we only have the formula with 3/2 in our notes, we don't have the one with the 5/2. I think maybe for simplicity we only consider 3 degrees of freedom (x, y, z).

So the calculation I did in my last post would be correct for a monatomic molecule? Also what exactly are the other 2 degrees of freedom that a diatomic molecule has?

Both main components of air, nitrogen and oxygen, form diatomic molecules. They are like dumbbells. Rotation about two axis, normal to the axis of symmetry of the molecule, contributes to the internal energy.